Bubble is formed and rides the sound wave to center of chamber (think of an eggcrate formed by sound waves). Then you tune the sound waves in such a manner that the bubble expands (pressure drops) during one phase and then collapses on itself during the compression phase. Once pressure increases (along with temperature), the gas will radiate. Temperature are hotter than the sun, but not high enough for fusion (has been tried about 10 years ago with deuterated water and acetone fluid). Seth putterman at ucla worked on this.